Savings on energy resources in various fields have attracted public attention in recent years, and as a result, an influence thereof has spread into many areas including, for example, a field of a power supply. Specifically, for example, there has been a demand for a switching power supply device to be even more efficient.
A power factor correction (PFC) converter that corrects the power factor of the switching power supply device includes a diode bridge that inputs common AC power and performs a full-wave rectification of the input AC power and a step-up chopper circuit that inputs the full-wave rectified voltage. In order to reduce power loss in a diode bridge circuit, there is also a switching power supply device serving as a bridgeless PFC without a rectifying bridge circuit.
FIG. 1 illustrates one example of a circuit diagram of a switching power supply device with a bridgeless PFC circuit. In the switching power supply device illustrated in FIG. 1, the common AC power is connected to a first input terminal A1 and a second input terminal A2, and AC input voltage VAC is input. A diode bridge that performs a full-wave rectification of the AC input voltage VAC is not provided in an input terminal of the switching power supply device.
A first serial circuit including a first switching element TR1 and a first diode D1 is connected to two output terminals P1 and P2 in parallel. Also, a second serial circuit including a second switching element TR2 and a second diode D2 is connected to two output terminals P1 and P2 in parallel. For example, a metal oxide semiconductor field effect transistor (MOSFET) may be used as a switching element.
A smoothing circuit by a condenser C1 that smoothes a DC output is connected to two output terminals P1 and P2 in parallel.
A first PFC circuit is constituted by a first inductor L1, the first switching element TR1, the first diode D1, and the condenser C1, which are connected in a T shape, and when the AC input voltage VAC is a plus cycle, the first PFC circuit serves as an active filter circuit that corrects the power factor of the power by reducing the distortion of a harmonic included in AC input current.
A second PFC circuit is constituted by a second inductor L2, a second switching element TR2, a second diode D2, and the condenser C1, which are connected in the T shape, and when the AC input voltage VAC is a minus cycle, the second PFC circuit serves as the active filter circuit that corrects the power factor of the power by reducing the distortion of the harmonic included in the AC input current.
The first inductor L1 is inserted between a connection point between the first switching element TR1 and the first diode D1, and the first input terminal A1 of the AC input power. The second inductor L2 is inserted between a connection point between the second switching element TR2 and the second diode D2, and the second input terminal A2 of the AC input power.
A first return diode D3 is inserted between a connection point between the first input terminal A1 and the first inductor L1, and a line of the output terminal P2. A second return diode D4 is inserted between a connection point between the second input terminal A2 and the second inductor L2, and a line of the output terminal P2.
FIGS. 2A and 2B are diagrams illustrating a current path of the switching power supply device illustrated in FIG. 1 when the AC input voltage VAC is a plus half-cycle. In the plus half-cycle, the PFC is controlled by turning ON and OFF the first switching element TR1. At that time, the second switching element TR2 is also turned ON and OFF simultaneously.
FIG. 2A illustrates a current path when the first switching element TR1 is turned ON and FIG. 2B illustrates a current path when the first switching element TR1 is turned OFF.
Referring to FIG. 2A, when the first switching element TR1 is turned ON, current that flows from the first input terminal A1 to the first inductor L1 flows from the first switching element TR1 to the line of the output terminal P2. While the current that flows to the line of the output terminal P2 returns to the second input terminal A2 through the return diode D4, the second switching element TR2 which is a MOSFET is also turned ON, and as a result, the voltage drop of the second switching element TR2 is smaller than voltage drop in the return diode D4. Thus, most of the return current flows to the second switching element TR2 and a small amount of current flows to the return diode D4.
Referring to FIG. 2B, when the first switching element TR1 is turned OFF, the current that flows from the first input terminal A1 to the first inductor L1 flows to the output terminal P1 not through the first switching element TR1 but through the first diode D1. The return current from the output terminal P2 returns to the second input terminal A2 through the return diode D4. The return current returns to the second input terminal A2 even through a parasitic diode (“a body diode”) BD2 of the second switching element TR2 and the second inductor L2.
Since the inductor has a property to allow current to continuously flow, the second inductor L2 allows the return current to continuously flow as it is even though the first switching element TR1 is switched to an OFF state from an ON state. Therefore, the return current continuously flows to the body diode BD2 of the second switching element TR2 as well.
The current does not flow on the return diode D4 once at the time when the first switching element TR1 is switched to the OFF state from the ON state and thereafter, the current slowly flows out. However, since the voltage drop of the body diode BD2 is smaller than that of the return diode D4, most of the return current flows to the body diode BD2 of the second switching element TR2 which is turned OFF and only a small amount of current flows to the return diode D4.
Power tends to be lost while the return current flows on the body diode BD2. Therefore, in order to further improve an output efficiency of the switching power supply device, the return current that flows to the body diode BD2 needs to decrease.
It may be considered that a high electron mobility transistor (GaN-HEMT) without a body diode is used as the first and second switching elements TR1 and TR2 in order to reduce loss in the body diode. However, in this case, a total of the return current flows to the return diode, and as a result, loss is generated in the return diode. The loss in the return diode becomes even larger than that of the body diode.
The following is reference documents:    [Document 1] Japanese Patent Application Laid-Open No. 2011-152017 and    [Document 2] Japanese Unexamined Patent Application Publication No. 2007-527687.